Clathrate Hydrate-Solid Surface Adhesive Shear Strength Measurements on Carbon Steel Surfaces.

Abstract

Oil and gas flowlines operating in subsea or cold terrestrial environments face the risk of forming hydrate deposits and plugs. The pressure differential across a hydrate plug can cause the plug to detach from the pipe wall and travel through the pipeline, potentially impacting a bend or inline equipment, causing damage or injury to personnel. Therefore, the hydrate-solid adhesive shear strength is of interest in estimating the maximum allowable differential pressure across a plug during depressurization procedures before the plug is likely to detach from the pipe wall. Quantifying the changes in adhesive shear strength with time and operational conditions is essential in estimating the potential of hydrate deposits to slough from the pipe wall. This work used a force meter with a cylindrical "pig-style" probe mounted to a single axis motorized test stand to measure the force required to dislodge model THF structure II hydrate plugs in carbon steel pipes. Profilometry measurements were used to quantify the mean surface roughness and relative peak frequency of the pipe surfaces. Contact angle measurements were performed of a water droplet on pristine, sanded, and corroded pipe surfaces immersed in nonpolar solvents. The adhesive shear strength of the hydrate plugs was measured for different subcoolings, solid surface roughness, and surface wettabilities. Subcooling was shown to impact the hydrate-solid adhesive shear strength, and a mixed adhesive/cohesive failure mechanism was observed at the highest subcooling tested. The surface roughness and wettability were also shown to influence the hydrate-solid adhesive shear strength, with readily wetting corroded and sanded carbon steel surfaces resulting in greater interface adhesive strength than the pristine carbon steel system. This is likely due to a Wenzel wetting mode at the interface, resulting in a higher ratio of actual to apparent wetted area as well as establishing a mechanical interlocking mechanism.